How to sort mongodb with pymongo

问题

I'm trying to use the sort feature when querying my mongoDB, but it is failing. The same query works in the MongoDB console but not here. Code is as follows:

import pymongo

from  pymongo import Connection
connection = Connection()
db = connection.myDB
print db.posts.count()
for post in db.posts.find({}, {'entities.user_mentions.screen_name':1}).sort({u'entities.user_mentions.screen_name':1}):
    print post

The error I get is as follows:

Traceback (most recent call last):
  File "find_ow.py", line 7, in <module>
    for post in db.posts.find({}, {'entities.user_mentions.screen_name':1}).sort({'entities.user_mentions.screen_name':1},1):
  File "/Library/Python/2.6/site-packages/pymongo-2.0.1-py2.6-macosx-10.6-universal.egg/pymongo/cursor.py", line 430, in sort
  File "/Library/Python/2.6/site-packages/pymongo-2.0.1-py2.6-macosx-10.6-universal.egg/pymongo/helpers.py", line 67, in _index_document
TypeError: first item in each key pair must be a string

I found a link elsewhere that says I need to place a 'u' infront of the key if using pymongo, but that didn't work either. Anyone else get this to work or is this a bug.

回答1:

.sort(), in pymongo, takes key and direction as parameters.

So if you want to sort by, let's say, id then you should .sort("_id", 1)

For multiple fields:

.sort([("field1", pymongo.ASCENDING), ("field2", pymongo.DESCENDING)])

回答2:

You can try this:

db.Account.find().sort("UserName")  
db.Account.find().sort("UserName",pymongo.ASCENDING)   
db.Account.find().sort("UserName",pymongo.DESCENDING)

回答3:

This also works:

db.Account.find().sort('UserName', -1)
db.Account.find().sort('UserName', 1)

I'm using this in my code, please comment if i'm doing something wrong here, thanks.

回答4:

Why python uses list of tuples instead dict?

In python you cannot guarantee that dictionary will be interpreted in the order you declared.

So, in mongo shell you could do .sort({'field1':1,'field2':1}) and the interpreter should sort field1 at first level and field 2 at second level.

If this sintax was used in python, there is a chance to sort field2 at first level. With tuple there is no risk.

.sort([("field1",pymongo.ASCENDING), ("field2",pymongo.DESCENDING)])

回答5:

.sort([("field1",pymongo.ASCENDING), ("field2",pymongo.DESCENDING)])

Python uses key,direction. You can use the above way.

So in your case you can do this

for post in db.posts.find().sort('entities.user_mentions.screen_name',pymongo.ASCENDING):
        print post

回答6:

TLDR: Aggregation pipeline is faster as compared to conventional .find().sort().

Now moving to the real explanation. There are two ways to perform sorting operations in MongoDB:

Using .find() and .sort().
Or using the aggregation pipeline.

As suggested by many .find().sort() is the simplest way to perform the sorting.

.sort([("field1",pymongo.ASCENDING), ("field2",pymongo.DESCENDING)])

However, this is a slow process compared to the aggregation pipeline.

Coming to the aggregation pipeline method. The steps to implement simple aggregation pipeline intended for sorting are:

$match (optional step)
$sort

NOTE: In my experience, the aggregation pipeline works a bit faster than the .find().sort() method.

Here's an example of the aggregation pipeline.

db.collection_name.aggregate([{
    "$match": {
        # your query - optional step
    }
},
{
    "$sort": {
        "field_1": pymongo.ASCENDING,
        "field_2": pymongo.DESCENDING,
        ....
    }
}])

Try this method yourself, compare the speed and let me know about this in the comments.

Edit: Do not forget to use allowDiskUse=True while sorting on multiple fields otherwise it will throw an error.

来源：https://stackoverflow.com/questions/8109122/how-to-sort-mongodb-with-pymongo

标签

python

mongodb

pymongo